Reversible Crosslinking of Polymer/Metal-Ion Complexes for a Microfluidic Switch.

The importance of chitosan has been strongly emphasized in literature because this natural polymer could not only remove heavy metal ions in water but also have the potential for recyclability. However, reversible phase transition and its dynamics, which are highlighting areas of a recycle process, have not been studied sufficiently. Here, we present dynamic studies of the dissolution as well as the gelation of a physically crosslinked chitosan hydrogel. Specifically, a one-dimensional gel growth system and an acetate buffer solution were prepared for the precise analysis of the dominant factors affecting a phase transition. The dissolution rate was found to be regulated by three major factors of the pH level, Cu2+, and NO2 -, while the gelation rate was strongly governed by the concentration of OH-. Apart from the gelation rate, the use of Cu2+ led to the rapid realization of gel characteristics. The results here provide strategies for process engineering, ultimately to determine the phase-transition rates. In addition, a microfluidic switch was successfully operated based on a better understanding of the reversible crosslinking of the chitosan hydrogel. Rapid gelation was required to close the channel, and a quick switchover was achieved by a dissolution enhancement strategy. As a result, factors that regulated the rates of gelation or dissolution were found to be useful to operate the fluidic switch.